Add Documentation/vm/multigen_lru.rst. Signed-off-by: Yu Zhao --- Documentation/vm/index.rst | 1 + Documentation/vm/multigen_lru.rst | 192 ++++++++++++++++++++++++++++++ 2 files changed, 193 insertions(+) create mode 100644 Documentation/vm/multigen_lru.rst diff --git a/Documentation/vm/index.rst b/Documentation/vm/index.rst index eff5fbd492d0..c353b3f55924 100644 --- a/Documentation/vm/index.rst +++ b/Documentation/vm/index.rst @@ -17,6 +17,7 @@ various features of the Linux memory management swap_numa zswap + multigen_lru Kernel developers MM documentation ================================== diff --git a/Documentation/vm/multigen_lru.rst b/Documentation/vm/multigen_lru.rst new file mode 100644 index 000000000000..cf772aeca317 --- /dev/null +++ b/Documentation/vm/multigen_lru.rst @@ -0,0 +1,192 @@ +===================== +Multigenerational LRU +===================== + +Quick Start +=========== +Build Options +------------- +:Required: Set ``CONFIG_LRU_GEN=y``. + +:Optional: Change ``CONFIG_NR_LRU_GENS`` to a number ``X`` to support + a maximum of ``X`` generations. + +:Optional: Change ``CONFIG_TIERS_PER_GEN`` to a number ``Y`` to support + a maximum of ``Y`` tiers per generation. + +:Optional: Set ``CONFIG_LRU_GEN_ENABLED=y`` to turn the feature on by + default. + +Runtime Options +--------------- +:Required: Write ``1`` to ``/sys/kernel/mm/lru_gen/enable`` if the + feature was not turned on by default. + +:Optional: Change ``/sys/kernel/mm/lru_gen/spread`` to a number ``N`` + to spread pages out across ``N+1`` generations. ``N`` should be less + than ``X``. Larger values make the background aging more aggressive. + +:Optional: Read ``/sys/kernel/debug/lru_gen`` to verify the feature. + This file has the following output: + +:: + + memcg memcg_id memcg_path + node node_id + min_gen birth_time anon_size file_size + ... + max_gen birth_time anon_size file_size + +Given a memcg and a node, ``min_gen`` is the oldest generation +(number) and ``max_gen`` is the youngest. Birth time is in +milliseconds. The sizes of anon and file types are in pages. + +Recipes +------- +:Android on ARMv8.1+: ``X=4``, ``N=0`` + +:Android on pre-ARMv8.1 CPUs: Not recommended due to the lack of + ``ARM64_HW_AFDBM`` + +:Laptops running Chrome on x86_64: ``X=7``, ``N=2`` + +:Working set estimation: Write ``+ memcg_id node_id gen [swappiness]`` + to ``/sys/kernel/debug/lru_gen`` to account referenced pages to + generation ``max_gen`` and create the next generation ``max_gen+1``. + ``gen`` should be equal to ``max_gen``. A swap file and a non-zero + ``swappiness`` are required to scan anon type. If swapping is not + desired, set ``vm.swappiness`` to ``0``. + +:Proactive reclaim: Write ``- memcg_id node_id gen [swappiness] + [nr_to_reclaim]`` to ``/sys/kernel/debug/lru_gen`` to evict + generations less than or equal to ``gen``. ``gen`` should be less + than ``max_gen-1`` as ``max_gen`` and ``max_gen-1`` are active + generations and therefore protected from the eviction. Use + ``nr_to_reclaim`` to limit the number of pages to be evicted. + Multiple command lines are supported, so does concatenation with + delimiters ``,`` and ``;``. + +Framework +========= +For each ``lruvec``, evictable pages are divided into multiple +generations. The youngest generation number is stored in ``max_seq`` +for both anon and file types as they are aged on an equal footing. The +oldest generation numbers are stored in ``min_seq[2]`` separately for +anon and file types as clean file pages can be evicted regardless of +swap and write-back constraints. Generation numbers are truncated into +``order_base_2(CONFIG_NR_LRU_GENS+1)`` bits in order to fit into +``page->flags``. The sliding window technique is used to prevent +truncated generation numbers from overlapping. Each truncated +generation number is an index to an array of per-type and per-zone +lists. Evictable pages are added to the per-zone lists indexed by +``max_seq`` or ``min_seq[2]`` (modulo ``CONFIG_NR_LRU_GENS``), +depending on whether they are being faulted in. + +Each generation is then divided into multiple tiers. Tiers represent +levels of usage from file descriptors only. Pages accessed N times via +file descriptors belong to tier order_base_2(N). In contrast to moving +across generations which requires the lru lock, moving across tiers +only involves an atomic operation on ``page->flags`` and therefore has +a negligible cost. + +The workflow comprises two conceptually independent functions: the +aging and the eviction. + +Aging +----- +The aging produces young generations. Given an ``lruvec``, the aging +scans page tables for referenced pages of this ``lruvec``. Upon +finding one, the aging updates its generation number to ``max_seq``. +After each round of scan, the aging increments ``max_seq``. + +The aging maintains either a system-wide ``mm_struct`` list or +per-memcg ``mm_struct`` lists, and it only scans page tables of +processes that have been scheduled since the last scan. Since scans +are differential with respect to referenced pages, the cost is roughly +proportional to their number. + +The aging is due when both of ``min_seq[2]`` reaches ``max_seq-1``, +assuming both anon and file types are reclaimable. + +Eviction +-------- +The eviction consumes old generations. Given an ``lruvec``, the +eviction scans the pages on the per-zone lists indexed by either of +``min_seq[2]``. It first tries to select a type based on the values of +``min_seq[2]``. When anon and file types are both available from the +same generation, it selects the one that has a lower refault rate. + +During a scan, the eviction sorts pages according to their generation +numbers, if the aging has found them referenced. It also moves pages +from the tiers that have higher refault rates than tier 0 to the next +generation. + +When it finds all the per-zone lists of a selected type are empty, the +eviction increments ``min_seq[2]`` indexed by this selected type. + +Rationale +========= +Limitations of Current Implementation +------------------------------------- +Notion of Active/Inactive +~~~~~~~~~~~~~~~~~~~~~~~~~ +For servers equipped with hundreds of gigabytes of memory, the +granularity of the active/inactive is too coarse to be useful for job +scheduling. False active/inactive rates are relatively high, and thus +the assumed savings may not materialize. + +For phones and laptops, executable pages are frequently evicted +despite the fact that there are many less recently used anon pages. +Major faults on executable pages cause ``janks`` (slow UI renderings) +and negatively impact user experience. + +For ``lruvec``\s from different memcgs or nodes, comparisons are +impossible due to the lack of a common frame of reference. + +Incremental Scans via ``rmap`` +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Each incremental scan picks up at where the last scan left off and +stops after it has found a handful of unreferenced pages. For +workloads using a large amount of anon memory, incremental scans lose +the advantage under sustained memory pressure due to high ratios of +the number of scanned pages to the number of reclaimed pages. On top +of that, the ``rmap`` has poor memory locality due to its complex data +structures. The combined effects typically result in a high amount of +CPU usage in the reclaim path. + +Benefits of Multigenerational LRU +--------------------------------- +Notion of Generation Numbers +~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +The notion of generation numbers introduces a quantitative approach to +memory overcommit. A larger number of pages can be spread out across +configurable generations, and thus they have relatively low false +active/inactive rates. Each generation includes all pages that have +been referenced since the last generation. + +Given an ``lruvec``, scans and the selections between anon and file +types are all based on generation numbers, which are simple and yet +effective. For different ``lruvec``\s, comparisons are still possible +based on birth times of generations. + +Differential Scans via Page Tables +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Each differential scan discovers all pages that have been referenced +since the last scan. Specifically, it walks the ``mm_struct`` list +associated with an ``lruvec`` to scan page tables of processes that +have been scheduled since the last scan. The cost of each differential +scan is roughly proportional to the number of referenced pages it +discovers. Unless address spaces are extremely sparse, page tables +usually have better memory locality than the ``rmap``. The end result +is generally a significant reduction in CPU usage, for workloads +using a large amount of anon memory. + +To-do List +========== +KVM Optimization +---------------- +Support shadow page table scanning. + +NUMA Optimization +----------------- +Support NUMA policies and per-node RSS counters. -- 2.31.1.295.g9ea45b61b8-goog